1. FIELD OF THE INVENTION
This invention relates to a process for preparing a polyurethane emulsion. More specifically, the present invention relates to a process for the preparation of a homogeneous, stable, amphoteric, self-emulsified, aqueous, polyurethane emulsion.
2. DESCRIPTION OF THE PRIOR ART
Various methods have been known for preparing polyurethane emulsions. A typical conventional method comprises reacting a polyhydroxyl compound with an excess of an organic polyisocyanate thereby to form an isocyanate-terminated urethane prepolymer, chain-extending the resulting urethane prepolymer by the use of a chain extender having a tertiary amino group thereby to form a polyurethane composition having a tertiary amino group, quaternizing the tertiary amino group and thereafter mixing the resulting product with water or an aqueous solution of an acid so as to obtain a so-called "self-emulsified" polyurethane composition containing no emulsifier.
According to another conventional method, a polyurethane emulsion can be prepared, for example, by dispersing a polyurethane composition in water in the presence of an emulsifier.
Generally, a polyurethane resin has excellent physical and chemical properties. However, the physical and chemical properties of a polyurethane resin film prepared from the polyurethane emulsion that is obtained by the above-mentioned known method are remarkably inferior in comparison with the properties of a cross-linked type polyurethane resin, partly because it is difficult to introduce chemical cross-linkages into said polymer and partly because the emulsifier remains in the polyurethane resin.
When a polyurethane polymer contains strong cross-linkages, handling of the polymer in the production procedures has generally been very difficult, and in most cases, impossible, because of the poor meltability and poor solubility properties of the polymer. It has therefore been almost impossible to prepare an emulsion by dispersing such a cross-linked polymer in water. Even if the polyurethane can be dispersed in water, the resulting polyurethane emulsion is extremely unstable.